The invention covers a process for the wet-mechanical separation of solids of varying density from solids mixtures, such as mixtures of household and industrial waste including plastics (PVC, PO, PS, PA, PET, PE, PP) which have been contaminated with heavy mineral solids such as sands, and/or with other heavy solids, such as metals, preferably for the separation of mixed plastic waste.
Pursuant to current environmental laws and regulations of many countries, plastic waste mixtures from household or industrial disposable product and packaging waste shall be 80% recycled. As to recycling of waste as a feedstock, this can be done by incineration, e.g., in a blast furnace, or as to material recycling in the manufacture (or remanufacture) of commodity goods. Heretofore, the easiest way to separate the waste mixture has been to do so manually into different fractions which are directed to varying use. The bottle and the plastic sheeting fraction are predominantly made up of HDPE and/or LDPE which after cleaning can be well utilized for material recycling.
The procedure of separation for different plastic phase or fractions is more difficult for mixed plastic waste because in addition to a multitude of different plastic types also composite materials and varying foreign matter exist as impurities in that waste. No more than approximately half of the waste can be removed as light plastic phase primarily made up of PE and PP--via separation by density and afterwards be used for material recycling. Therefore, it is general practice today to have the mixed plastic waste exclusively used for feedstock recycling after separation of approximately 10% of impurities (metals, etc.). These impurities are normally removed by simple mechanical processes, such as air classification, screening, magnetic separation, etc.
In view of the fact, however, that material recycling is the less polluting recycling method, efforts should be made to recover an as large portion of the light plastic phase as possible from the mixed plastic waste for material recycling. Consequently, two separation procedures have to be carried out to reach that objective, i.e., separation of the light plastic (for material recycling) and separation of impurities from the remaining heavy plastic phase (for feedstock recycling). However, the methods used so far make that process very complicated and, consequently, uneconomical.